The essential function of the optical pick-up system is to detect an audio signal recorded on the disk and produce an electric signal corresponding to the recorded information. The system generates a control signal for producing the signal. Further, the system has an optical system for detecting a focus error signal and a tracking error signal.
FIGS. 9a and 9b show a beam for detecting a tracking error signal using a push-pull method. A laser light from a laser diode (not shown) is focused onto a disk 1 through an objective 2 and a reflected light 3 is focused on a photo detector 4. The reflected light 3 includes an image detecting light and a tracking error detecting light. In a conventional system, the photodetector 4 for detecting tracking error signal is divided into two detectors PD1 and PD2. Therefore the reflected light from the disk 1 is divided into two parts on the photodetectors PD1 and PD2, respectively, for comparing the intensities of the divided light with each other. If there is no error in tracking, the intensities of the divided light are the same. Thus, the spots of beams on the photodetectors PD1 and PD2 are symmetrical about the optical axis between the photodetectors as indicated by a dashed line of FIG. 9b.
On the other hand, the optical axis XI of the tracking error detecting light 3a and the optical axis XII of the image detecting light 3b may deflect by a difference .alpha. because of the movement of a tracking actuator (not shown). Consequently, the illuminated spot 3A on the photodetector PD1 is different from the spot 3B on the photodetector PD1 in area. As a result, an offset is included in the tracking error signal.
More particularly, if the output powers of the photodetectors PD1 and PD2 are 3A.sub.0 and 3B.sub.0 when no offset occur, the tracking error signal TE is EQU TE=3A.sub.0 -3B.sub.0 =0
If the offset quantity is .alpha., and the output powers are 3A.sub.1 and 3B.sub.1 when an offset occurs, the tracking error signal TE is EQU TE=3A.sub.1 -3B.sub.1 =(3B.sub.0 +.alpha.)-(3A.sub.0 -.alpha.)=2.alpha.
Accordingly, the pickup is moved by a power corresponding to the tracking error 2.alpha. so that the tracking error may become zero. Namely, the pickup malfunctions because there is no tracking error.
Further, when the disk 1 is tilted relative to the beam as the disk rotates as shown is FIG. 9c, an offset also occurs in the tracking error signal.
In order to reduce the offset in the tracking error signal, the diameter of the spot on the photodetector is reduced so as to be smaller than the diameter of the reflected beam for preventing the influence of the offset. By this system, about half of the tracking error signal is suppressed.
However, in this system, since a small part of the reflected light is used as the effective signal, S/N ratios of both of the tracking error signal and an RF signal reduce. The reduction of the S/N ratio deteriorates characteristic of signals such as the servo signal and RF signal to be reproduced.